The present invention is concerned with improving efficiency of braking performance in vehicle braking systems, in particular within the context of vehicle braking systems having electro-hydraulic (EHB) braking.
A typical EHB system for a vehicle comprises a brake pedal, respective braking devices which are connected to the vehicle wheels and which are capable of being brought into communication with electronically controlled control valves in order to apply hydraulic fluid under pressure to the braking devices, a hydraulic pump driven by an electric motor, and a high pressure hydraulic pressure accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the electrically controlled valves in order, in so called xe2x80x9cbrake-by-wirexe2x80x9d mode, to apply hydraulic fluid under pressure to the braking devices in proportion to the driver""s demand as sensed at the brake pedal. The EHB system is controlled by an electronic controller (ECU). In the case of typical four-wheeled vehicles, there are four said braking devices at the two front wheels and two rear wheels of the vehicle, respectively.
In order to enable the vehicle to be braked in conditions where for some reason the EHB system has become inoperative, for example because of a major component failure, it is usual in vehicles fitted with EHB to include a mechanical back-up system comprising a master cylinder which is linked to the brake pedal and which can be arranged to be coupled hydraulically to respective brake actuators at the front wheels to provide at least some braking in the event of total EHB failure.
This is known as the xe2x80x9cpush-throughxe2x80x9d mode of braking. In some circumstances, it is preferable for the master cylinder to be in the form of a tandem cylinder coupled to the brake actuators of the two front wheels of the vehicle by way of separate pressure applicator channels.
In order to make the EHB system xe2x80x9cfeelxe2x80x9d like a conventional braking system in the normal EHB mode of braking, a travel simulator is also usually provided which is connected hydraulically to the master cylinder coupled to the brake pedal and which allows, by increasing the volume under pressure, the brake pedal to be depressed to an extent comparable with that of conventional systems.
It is already known that if a major problem occurs in the EHB system operating the front brakes so that push-through operation at the front brakes is selected, but the EHB system of the rear brakes is still secure, then the rear brakes should continue to be controlled under EHB, but using a pressure-based demand signal based upon the push-through pressure in the front brakes, However, this arrangement has the potential problem that, if there is a hydraulic failure in the master cylinder or one front brake, the master cylinder pressure may be zero so that the rear brake demand for the EHB-controlled rear brakes will also be zero.
It is an object of the present invention to mitigate this problem.
In accordance with the present invention, in the situation where push-through operation has been selected at the front brakes but the rear brakes are continuing to be operated under EHB, a rear brake demand is established using a combination of the individually measured pressures in both of the front brake actuator circuits and at the master cylinder.
Advantageously, in order to cater for situations where the observed pressures differ, the rear brake demand is calculated as a xe2x80x9cmultiple averagexe2x80x9d from the expression:
[P2+(P1+P3)/2]/2
where P1 and P3 are respectively signals from the master cylinder and a front brake sensor in the same push-through circuit and P2 is a signal from the front brake sensor in the other circuit.
A pedal travel signal can be used to confirm that travel is in the expected range, but a precise reading is not required. Normal pedal-sensor-cross-check errors are preferably suppressed. The travel simulator will preferably also be isolated in this mode.
In a preferred embodiment, EHB control of the rear axle is achieved via a special demand table that uses only the master-cylinder pressure sensor and the two front-brake-pressure sensors.
Critical fault-scenarios determining this formula include high-in-range faults at the master-cylinder pressure (P1), and in some cases leakage from/air in the secondary circuit,
For this latter scenario it may be that only one front wheel is braked, and the multiple-average pressure will then be less than the pressure in that brake. To ensure that statutory secondary-braking performance is still available in this case, the demand table should be arranged with a gain factor  greater than 1, eg. 2, between the multiple-average control pressure and the rear-brake pressure. This should allow the smaller rear-axle brakes to compensate for the missing front-wheel drag.
In some embodiments, the rear brake pressure can be set up to its maximum, similar to the known Brake Assist (BA) function, and can then be controlled via the ABS to maintain the vehicle stability.
In the case that only one front wheel can be braked in the push-through mode then, for vehicle stability reasons, only the rear wheel which is diagonally opposite to the braked front wheel is braked, at least occasionally.
The latter two situations can be improved further by utilising sensor information from the known Vehicle Stability Control (VSC), eg. the steering angle, the transversal acceleration, or the yaw moment. By this means, it is possible to distinguish if the vehicle is cornering or driving straight and it can be forseen when the vehicle stability is becoming critical.
In an EHB system when a type of failure occurs that needs the system to revert to push-through in some failure cases it is possible to retain the power braking function on the rear axle. In this case, ABS may be retained on the rear axle provided that there is enough wheel speed data from all four wheels. The retention of ABS in this case is a benefit for stability.
If the surface conditions and/or the push-through pressure generated by the driver causes front wheel lock, the two front wheel speed levels are now zero and to retain the ABS on the rear axle results in instability on some surfaces as the slip levels on the rear axle may be excessive (caused by the paucity of wheel speed data). This instability results in spinning out of lane.
In the latter circumstances, two possibilities may be considered as follows:
When the front wheels lock under the above stated conditions, it is arranged for one rear wheel to be under braked to retain a good vehicle speed reference. or
Increase the pressure in the rear brakes to ensure that they lock. This will give a good deceleration and will ensure that the car does not pull out of a straight line.
The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, wherein: